Cross point memory array with memory plugs exhibiting a characteristic hysteresis

ABSTRACT

Providing a cross point, memory array with memory plugs exhibiting a characteristic hysteresis. The memory plugs exhibit a hysteresis that, in the low resistive state, the first write threshold voltage is the point above which any voltages applied across the memory plug have substantially no effect on the resistive state and below which a voltage pulse will alter the resistance of the memory plug. Similarly, in the high resistive state, the second write threshold voltage is the point below which any voltages applied across the memory plug have substantially no effect on the resistive state and above which a voltage pulse will alter the resistance of the memory plug. The read voltages applied to the memory plug are typically above the first write threshold voltage and lower than the second write threshold voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/400,849, filed Aug. 2, 2002, the U.S. Provisional Application No.60/422,922, filed Oct. 31, 2002, and the U.S. Provisional ApplicationNo. 60/424,083, filed Nov. 5, 2002, all of which are incorporated hereinby reference in their entireties and for all purposes. This applicationis related to sister applications entitled “Cross Point Memory ArrayUsing Multiple Thin Films,” application Ser. No.: 10/330,512, filed Dec.26, 2002, now Pat. No. 6,753,561; “Cross Point Memory Array UsingMultiple Modes of Operation,” application Ser. No.: 10/330,153, filedDec. 26, 2002; “Cross Point Memory Array Using Distinct Voltages,”application Ser. No.: 10/330,964, filed Dec. 26, 2002; “Providing AReference Voltage To A Cross Point Memory Array,” application Ser. No.:10/330,170, filed Dec. 26, 2002; “Multi-Output Multiplexor,” applicationSer. No.: 10/330,150, filed Dec. 26, 2002; and “Multiplexor Having AReference Voltage On Unselected Lines,” application Ser. No. 10/330,965files Dec. 26, 2002; all of which were filed on hereby incorporatedherein by reference in their entireties and for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to memory, and more specificallyto memory employing a cross point array.

2. Description of the Related Art

Conventional nonvolatile memory requires three terminal MOSFET-baseddevices. The layout of such devices are not ideal, usually requiringfeature sizes of 8f² for each memory cell, where f is the minimumfeature size.

However, not all memory elements require three terminals. Certaincomplex metal oxides (CMOs), for example, can retain a resistive stateafter being exposed to an electronic pulse, which can be generated fromtwo terminals. U.S. Pat. No. 6,204,139, issued Mar. 20, 2001 to Liu etal., incorporated herein by reference for all purposes, describes someperovskite materials that exhibit such characteristics. The perovskitematerials are also described by the same researchers in“Electric-pulse-induced reversible resistance change effect inmagnetoresistive films,” Applied Physics Letters, Vol. 76, No. 19, 8 May2000, and “A New Concept for Non-Volatile Memory: The Electric-PulseInduced Resistive Change Effect in Colossal Magnetoresistive ThinFilms,” in materials for the 2001 Non-Volatile Memory TechnologySymposium, all of which are hereby incorporated by reference for allpurposes.

Similarly, the IBM Zurich Research Center has also published threetechnical papers that also discuss the use of metal oxide material formemory applications: “Reproducible switching effect in thin oxide filmsfor memory applications,” Applied Physics Letters, Vol. 77, No. 1, 3Jul. 2000, “Current-driven insulator-conductor transition andnonvolatile memory in chromium-doped SrTiO₃ single crystals,” AppliedPhysics Letters, Vol. 78, No. 23, 4 June 2001, and “Electric currentdistribution across a metal-insulator-metal structure during bistableswitching,” Journal of Applied Physics, Vol. 90, No. 6, 15 September2001, all of which are hereby incorporated by reference for allpurposes.

Similarly, magnetic RAM (MRAM) requires only two terminals to deliver amagnetic field to the memory element. Other two terminal devices includeOvonic Unified Memory (OUM), which uses chalcogenic layers of material,and various types of ferroelectric memory. With only two terminals, ithas been theorized that memory can be arranged in a cross pointarchitecture.

However, mere recognition that a two terminal memory element istheoretically capable of being placed in a cross point array does notsolve many of the non-trivial problems associated with actually creatingsuch a device.

SUMMARY OF THE INVENTION

The present invention provides a cross point memory array. In oneembodiment, the memory array includes a first layer of conductive arraylines, a second layer of conductive array lines and a plurality ofmemory plugs.

The first layer of conductive array lines is arranged so that they donot come into direct contact with each other. Similarly, the secondlayer of conductive array lines is arranged so that they do not comeinto direct contact with either each other or any of the conductivearray lines of the first layer. The plurality of memory plugs arelocated at the intersections of the first layer of conductive arraylines and the second layer of conductive array lines. Each memory plugis in electrical contact with one of the conductive array lines from thefirst layer and one of the conductive array lines from the second layersuch that each memory plug is associated with a unique pair ofconductive array lines.

Additionally, each memory plug has a low resistive state and a highresistive state. The resistive state of the memory plug is capable ofbeing determined when a read voltage is applied across the unique pairof conductive array lines.

Furthermore, each memory plug exhibits a hysteresis that ischaracterized by a first write threshold when the memory plug is in thelow resistive state and a second write threshold when the memory plug isin the high resistive state. The first write threshold voltage isdefined as the point where voltages applied across the unique pair ofconductive array lines that are higher than the first write thresholdhave substantially no effect on the resistive state of the memory plug.Similarly, the second write threshold voltage is defined as the pointwhere voltages applied across the unique pair of conductive array linesthat are lower than the second write threshold voltage havesubstantially no effect on the resistive state of the memory plug.

Alternatively, the hysteresis can be characterized by the mirror imageof the previously described hysteresis. In other words, the first writethreshold voltage is defined as the point where voltages applied acrossthe unique pair of conductive array lines that are lower than the firstwrite threshold have substantially no effect on the resistive state ofthe memory plug. The second write threshold voltage is defined as thepoint where voltages applied across the unique pair of conductive arraylines that are higher than the second write threshold voltage havesubstantially no effect on the resistive state of the memory plug.

In a preferred embodiment, the read voltage is higher than the firstwrite threshold voltage and lower than the second write thresholdvoltage. Applying read voltage therefore, has substantially no effect onthe resistive state of the memory plug.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 depicts an exemplary cross point memory array in a perspectiveview;

FIG. 2 depicts an exemplary memory plug in contact with two conductivearray lines in a side view;

FIG. 3 depicts the exemplary cross point memory array in a plan view;

FIG. 4 is a graph illustrating exemplary IV characteristics of a memoryplug;

FIG. 5A is a block diagram depicting the logical connections of varioussystems that contribute to selecting a memory plug;

FIG. 5B is a block diagram depicting the physical connections of thevarious systems that contribute to selecting a memory plug;

FIG. 6A is a block diagram depicting one mechanism for preventing theunselected conductive array lines from floating to an undesired voltage;

FIG. 6B is a block diagram depicting another mechanism for preventingthe unselected conductive array lines from floating to an undesiredvoltage;

FIG. 6C is a block diagram depicting an improvement on the mechanism forpreventing the unselected conductive array lines from floating to anundesired voltage depicted in FIG. 6B;

FIG. 6D is a block diagram depicting an improvement on the mechanism forpreventing the unselected conductive array lines from floating to anundesired voltage depicted in FIG. 6C;

FIG. 6E is a block diagram depicting yet another mechanism forpreventing the unselected conductive array lines from floating to anundesired voltage by using a 3-output driver;

FIG. 7 is a block diagram depicting one possible configuration of a3-output driver;

FIG. 8A is a graph illustrating exemplary RV characteristics of a memoryelement initially biased in one direction

FIG. 8B is a graph illustrating exemplary RV characteristics of a memoryelement initially biased in the other direction;

FIG. 9 is an exemplary flow chart of various processing steps that couldbe involved in a cross point array; and

FIG. 10 is an exemplary memory cell that is formed using the processingsteps in a side view.

It is to be understood that, in the drawings, like reference numeralsdesignate like structural elements. Also, it is understood that thedepictions in the figures are not necessarily to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts an exemplary cross point array 100. A bottom layer ofx-direction conductive array lines 105 and a top layer of y-directionconductive array lines 110 sandwiches a plurality of memory plugs 115.The conductive array lines 105 and 110 are arranged in the cross pointarray 100 so that each individual memory plug is capable of beinguniquely identified and, therefore, uniquely selected by a singlex-direction conductive array line and a single y-direction conductivearray line.

Conductive array lines 105 and 110 can be constructed of any conductivematerial, such as aluminum, copper, tungsten or certain ceramics.Ideally, the material will withstand a high-temperature fabricationprocess, have a low inherent resistance, and have a low cost. Often,however, compromises will have to be made.

For example, if the memory element is a CMO, very high temperaturesmight be required to form a polycrystalline or a single crystallinestructure. Depending upon the fabrication process (e.g., solution basedspin on followed by high temperature anneal, pulsed laser deposition,sputtering, and metalorganic chemical vapor deposition) the fabricationtemperature might require that refractory metals be used for conductivearray lines. However, refractive array lines have higher resistances,which means that a given x-direction conductive array line would not beable to timely access as many y-direction conductive array lines as alower resistance conductive array line, reducing the number of memorycells on the cross-point array 100. Therefore, compromises must be madein order to balance cost, size, resistivity and preferred fabricationtechniques. Typically, a conductive array line would cross between 64and 8192 perpendicular conductive array lines. Although the x-directionand y-direction conductive array lines can be of equal lengths (forminga square cross point array) they can also be of unequal lengths (forminga rectangular cross point array).

Each memory plug in the plurality of memory plugs 115 contains a memoryelement along with any other materials that may be necessary, such as aninsulating layer in an MRAM or a seed layer in a CMO. Additionally, aCMO memory plug would preferably include a non-ohmic device, as isdescribed in co-pending application “High Density NVRAM,” U.S.application Ser. No. 10/160,321, filed May 31, 2002. The non-ohmicdevice exhibits a very high resistance regime for a certain range ofvoltages (V_(NO−) to V_(NO+)) and a very low resistance regime forvoltages above and below that range.

FIG. 2 depicts a side view of an exemplary memory plug 205 with sevenseparate thin-film layers and sandwiched between two conductive arraylines 210 and 215. The seven layers are: an electrode layer 220, a layerof CMO material 225 (providing the memory element), another optionalelectrode layer 230, three layers that make up a metal-insulator-metal(MIM) structure 235, 240 and 245 (providing the non-ohmic device), andan optional final electrode 250.

The electrode layers 220, 230 and 250 are only necessary to the extentfabrication methods require them. Therefore, electrode layers 220, 230and 250 would ideally be as thin as possible while still preventingmetal inter-diffusion and, if necessary, being useful as a seed layer.Typical electrode layers 220,230 and 250 commonly used in fabricationinclude Pt, Au, Ag and Al. If the electrode layers 220, 230 and 250 areused only as a barrier to prevent metal inter-diffusion, then a thinlayer of metal, e.g. TiN, could be used. Any number of electricallyconductive materials can be used for CMO material's 225 seed layer 220.For example, the seed layer 220 could be a conductive perovskite, suchas LaNiO₃ or SrRuO₃ on Pt, a conductive metal oxide, such as IrO₂ on Iror RuO₂ on Ru, a noble metal such as Pt on TiN. To make fabrication moreefficient, the other electrode layers 230 and 250 could use the sameconductive layer that was used on the seed layer 220.

The CMO material 225 thickness is dictated by the desired resistivestates of the CMO material 225 and the write threshold voltage(V_(Wth)). Although very dramatic differences (e.g., 1000 timesdifference) between a low resistance state (R₁) and a high resistivestate (R₀) could be sensed very easily, the voltage that drives suchresistances would be less than ideal. Since large currents can bedestructive to semiconductors fabricated to small dimensions, no morethan 10 μA would be desired for a memory circuit in most cases.Therefore, a modest difference between R₀ and R₁ would typically be abetter choice. For example, if 1 volt were used as a read voltage(V_(R)), R₁ might be about 100 kΩ and R₀ might be about 1MΩ, making thecurrent either 10 μA or 1 μA, depending on the resistive state. Once aV_(R) is identified, a desired write voltage (V_(W)) can be determined.Not only should V_(W) be greater than V_(R), but it should also be farenough away from V_(R) to allow minor voltage fluctuations (e.g., due tofabrication imperfections) to have a negligible effect on the CMOmaterial 225. Similarly, V_(W) should be greater than V_(Wth), for thesame reason. A typical V_(W) might be about 2 volts, and V_(W) th mightbe about 1.5 volts. A CMO material 225 with a V_(Wth) of 1.5 volts mightbe between 1000 Å and 3000 Å thick.

The CMO material 225 will generally be a crystalline or polycrystallineperovskite structure. Generally, the CMO material 225 includes two ormore metals, the metals being selected from the group consisting oftransition metals, alkaline earth metals and rare earth metals. The CMOmaterial 225 can be any number of compositions, including manganites(e.g., Pr_(0.7)Ca_(0.3)MnO₃, Pr_(0.5)Ca_(0.5)MnO₃ and other PCMOs,LCMOs, etc.), titanites (e.g., STO:Cr), zirconates (e.g., SZO:Cr,Ca₂Nb₂O₇:Cr, and Ta₂O₅:Cr), and high Tc superconductors (e.g., YBCO).Specifically, MnO₃, when combined with the rare earth metals La, Pr orsome combination thereof and the alkaline earth metals Ca, Sr or somecombination thereof have been found to produce a particularly effectiveCMO material 225 for use in a memory plug 205.

The properties of the MIM structure 235, 240 and 245 (an example of anon-ohmic device) would then be dependant on ½ V_(R). FIG. 3 illustratesselection of a cell in the cross point array 100. The point ofintersection between a single x-direction conductive array line 305 anda single y-direction conductive array line 310 uniquely identifies asingle memory plug 315. However, the selected conductive array lines 305and 310 deliver a voltage to every memory plug associated with thoseconductive array lines. Therefore, the non-ohmic device must be able toblock current of the maximum voltage that will be seen on a singleconductive array line but pass current from the minimum voltage seen ona selected memory plug. If each conductive array line 305 and 310supplies half the required voltage of each operation, then ½ V_(W)′would be the maximum voltage on a single conductive array line andV_(R)′ would be the minimum voltage seen on a selected memory plug.V_(W)′ is V_(W) plus all other voltage drops in the memory plug 315(e.g., V_(NO)+) and V_(R)′ is V_(R) plus all other voltage drops in thememory plug 315.

If the non-ohmic device is the only other element that causes a voltagedrop, the minimum V_(NO+) can be calculated from V_(NO+)=½ V_(W)′=½(V_(NO+)+V_(W))=V_(W)). A V_(NO+) of 2V would cause V_(W)′ to be 4V andV_(R)′ to be 3V. However, a higher V_(NO+) might be appropriate to allowfor some fabrication inconsistencies and other additional elements inthe memory plug that provides a voltage drop. For example, with aV_(W)=2V, a V_(NO+) of 4V would cause V_(W)′ to be at least 6V andV_(R)′ to be at least 4V. Once a V_(NO+) is selected, then the thicknessof the non-ohmic device (e.g., MIM 235, 240 and 245) can be calculated.FIG. 4 illustrates exemplary IV characteristics of a memory plug.

The metal portion 235 and 245 of the MIM can be made of any conductivematerial. However, to make fabrication more efficient, the metal portion235 and 245 of the MIM could use the same conductive layer that was usedwith the seed layer 220. Therefore, metal layers 235 and 245 of the MIMcould also serve as the electrode layers 230 and 250, removing the needto have a separate electrode layers 230 and 250. The insulator layer 240of the MIM can be realized with Ta₂O₅, an amorphous SiC layer, or othersuch materials.

Other structures that might be used for the non-ohmic device include atleast two oppositely oriented diodes. The oppositely oriented diodes canbe connected either in series or in parallel.

When two diodes are in series, one diode's forward current is blocked bythe other diode at low voltages. However, at the breakdown voltage ofeach diode, the resistance to current flow diminishes greatly. Hence, inthis embodiment, it is the diodes' breakdown voltages that defineV_(NO+) and V_(NO−).

When the diodes are in parallel, multiple diodes could be used in adiode ring formation in order to have an appropriate forward biasvoltage drop. For example, if each diode where to have a forward biasvoltage drop of 0.7V, placing two diodes in series oriented in the samedirection would produce a total forward bias voltage drop of 1.4V.Similarly, a chain of two diodes in parallel with and oppositelyoriented to the previous two diodes would cause the entire non-ohmicdevice to exhibit a high resistance from −1.4V to +1.4V.

Yet another structure that might be used was described in a paper called“A switching Device based on a-Si:H N-i-dp-i-n stacked structure:Modeling and characterization,” by Domenico Caputo and Giampiero deCesare (IEEE Transactions on Electron Devices, Vol 43, No 12, December1996), incorporated herein by reference for all purposes. The five layerstructure described in the paper consists of an n-type silicon layer, anintrinsic silicon layer, a very thin p-type silicon layer, an intrinsicsilicon layer and an n-type layer.

Those skilled in the art will appreciate that the above-describedvoltages are voltages seen by the memory plug 315, and not necessarilythe absolute values of the voltages from ground. For example, placing 10volts on the selected x-direction conductive array line 305 and 6 voltson the selected y-direction conductive array line 310 would still attaina 4-volt drop. If the voltage drop across any single unselected memoryplug was to be no more than 3 volts, then unselected conductive arraylines 320, 325, 330, 335 and 340 would need to be held somewhere between7 volts and 9 volts in the above example.

Preferably, the unselected conductive array lines 320, 325, 330, 335 and340 would be held at a constant voltage, regardless of whether a read orwrite operation was being performed. Therefore, if the selectedx-direction conductive array line 310 were placed at 12 volts, then they-direction unselected conductive array lines 320 and 325 would need tobe held at 9 volts in the above example in order to keep the maximumvoltage drop to 3 volts. If symmetry was desired, a write operationmight require the x-direction conductive array line 310 be held to 11volts , the y-direction conductive array line 305 be held to 5 volts andall the unselected conductive array lines 320, 325, 330, 335 and 340 beheld to 8 volts (halfway between the voltages of the two conductivearray lines 305 and 310 for both read and write operations).

As illustrated in the preferred embodiment of FIGS. 5A and 5B, eachconductive array line 305, 310, 320, 325, 330, 335 and 340 is associatedwith a pass device 505, 510, 515, 520, 525, 530 and 535 (one n-type andone p-type transistor). The gate voltages of each transistor areadjusted such that the selected conductive array lines 305 and 310 areallowed to pass a voltage from the drivers 540. The voltage that ispassed can either be the full voltage from the drivers 540 during awrite operation (a magnitude of 3 volts in the above example) in a firstselect mode, a partial voltage during a read operation (a magnitude of 2volts in the above example) in a second select mode, or no voltage forthe unselected lines in an unselect mode. Therefore, the pass devices505 can act as part of the decoding circuitry as a type of modulatingcircuit, allowing one conductive array line to be on (“selected”) ateither a read or write voltage and the other conductive array lines tobe off (“unselected”). FIG. 5A illustrates the logical connections andFIG. 5B illustrates the same connections, but with a different physicallayout (the pass devices alternating on each end of the conductive arraylines).

Alternatively, in place of the pass device 505, 510, 515, 520, 525, 530and 535, a single transistor can be used. However, the transistor's gatevoltage would need to be such that it could pass large magnitudevoltages. For example, an n-channel transistor would pass the full 3V ifits gate were held to 3V plus the n-channel transistor's thresholdvoltage. A single transistor could still act as a modulating circuit byonly partially turning on the transistor.

In another embodiment, the modulation is performed upstream of the gatecircuit. Regardless of whether the gate circuit is a pass device or asingle transistor, the input to the gate circuit would already be at anappropriate read or write voltage. In this embodiment, the gate circuitwould only have a first select mode (passing substantially the fullinput voltage) and an unselect mode (passing substantially no voltage).

It should be noted that if the unselected conductive array lines 320,325, 330, 335 and 340 were not held to a certain voltage, they would beconsidered floating, which is generally discouraged in circuit design.Specifically, a problem would arise if, for example, an unselectedy-direction conductive array line 340 were floating at −3 volts. If theselected x-direction conductive array line 305 was at 3 volts and theselected y-direction conductive array line 310 was at −3 volts, twocells 315 and 545 would see a 6-volt drop, potentially disturbing theresistive states of both cells.

FIG. 6A illustrates one mechanism for preventing the unselectedconductive array lines 320, 325, 330, 335 and 340 from floating to anundesired voltage. Prior to a read or write operation, each pass device505, 510, 515, 520, 525, 530 and 535 would have both of theirtransistors turned on, the drivers 540 would be deactivated, and voltagereference generators 605 would be at some reference voltage (presumed tobe ground for the remainder of this document). Therefore, all theconductive array lines 305, 310, 320, 325, 330, 335 and 340 would bepulled to the reference voltage. During the read or write operation, thevoltage reference generator 605 would be deactivated, the driver 540would be activated, and only the appropriate pass devices 505 and 510would be on, bringing the selected conductive array lines 305 and 310 tothe driver voltage. Although the unselected conductive array lines 320,325, 330, 335 and 340 would be floating during a read or writeoperation, their parasitic capacitance might keep them at the referencevoltage for the time it takes to do a normal read or write cycle.However, lateral coupling capacitance will work against the parasiticcapacitance, raising the voltage on the unselected conductive arraylines 320, 325, 330 and 335 adjacent to the selected conductive arraylines 305 and 310.

FIG. 6B illustrates another mechanism for preventing the unselectedconductive array lines 320, 325, 330, 335 and 340 from floating to anundesired voltage. Each conductive array line 305, 310 320, 325, 330,335 and 340 would have an associated grounding pass device 610, 615,620, 625, 630, 635 and 640 that allows the conductive array lines 305,310, 320, 325, 330, 335 and 340 to be pulled to ground when activated.

The grounding pass devices 610, 615, 620, 625, 630, 635 and 640 could beactivated prior to a read or write operation (similar to the operationdescribed in connection with FIG. 6A), could be triggered by theselection of a conductive array line, or could always be kept on. If thegrounding pass devices 610, 615, 620, 625, 630, 635 and 640 were alwayson, their relative size would need to be small in relation to thedecoding pass devices 505, 510, 515, 520, 525, 530 and 535 so that theleakage to ground would have a small effect.

If the grounding pass devices 610, 615, 620, 625, 630, 635 and 640 weretriggered by the selection of a conductive array line, the process ofselecting the x-direction conductive array line 305, would activate thedecoding pass device 505 and deactivate the grounding pass device 610associated with the selected conductive array line 305 whiledeactivating the decoding pass devices 515 and 520 and activating thegrounding pass devices 635 and 640 associated with the unselectedconductive array lines 320 and 325. Similarly, selecting the y-directionconductive array line 310 would activate the decoding pass device 510and deactivate the grounding pass device 615 associated with theselected conductive array line 310 while deactivating the decoding passdevices 525, 530 and 535 and activating the grounding pass devices 620,625 and 630 associated with the unselected conductive array lines 330,335 and 340.

FIG. 6C is an improvement on FIG. 6B. However, instead of pass devices610, 615, 620, 625, 630, 635 and 640, a single transistor 645, 650, 655,660, 665, 670 and 675 is used. Assuming the transistors 645, 650, 655,660, 665, 670 and 675 were n-channel devices, they would be on only ifthe gate voltage were held to at least the threshold voltages of thetransistors. Similarly, if the transistors were p-channel devices, theywould be on if the gate voltages were held to at least the negativethreshold voltages. Therefore, an appropriate gate voltage can fullydischarge the conductive array lines.

The single n-channel (or p-channel) transistor embodiment can be usedeither at the beginning of a cycle or by using the inverse of the signalthat activates the n-channel (or p-channel) portion of the decoding passdevices 505, 510, 515, 520, 525, 530 and 535. As shown in FIG. 6D, insome embodiments, the inverse signal might simply be the p-channel (orn-channel) portion of the decoding pass devices 505, 510, 515, 520, 525,530 and 535.

FIG. 6E illustrates yet another mechanism for preventing the unselectedconductive array lines 320, 325, 330, 335 and 340 from floating to anundesired voltage by using a 3-output driver 680. Since the 3-outputdriver 680 would deliver a voltage to all of the decoding pass devices505, 510, 515, 520, 525, 530 and 535, such a mechanism could not be usedwhile a specific conductive array line was being selected. Therefore,the 3-output driver 680 could either be used to discharge the floatingvoltages prior to a read or write operation. Alternatively, the 3-outputdriver 680 could be used during a read or write operation if it wereonly used in connection with unselected banks, assuming multiple3-output drivers 680 are used for multiple banks.

FIG. 7 illustrates one possible configuration of a 3-output driver 680.A p-channel transistor 705 and an n-channel transistor 710 are arrangedin series. The n-channel transistor may be in a separate well from theCMOS logic because the substrate is tied to a negative voltage. Thesource of the p-channel transistor 705 is connected to a voltage sourceof +½ V_(W)′ and the source of the n-channel transistor 710 is connectedto a voltage source of −½ V_(W)′. The drains of both transistors areconnected to both the decoding pass devices 505, 510, 515, 520, 525, 530and 535 and transistor to ground 815. In order to discharge the entireline, the transistor to ground 715 would need to have an appropriategate voltage, as described in connection with FIG. 6C, or be a passdevice. The 3-output driver 680 would function when only one transistor705, 710 or 715 was on, while the other two transistors were off.

Depending on the embodiment, the driver 540 or the 3-output driver 680,must be able to supply either +½ V_(W)′ or −½ V_(W)′ to the decodingpass devices 505, 510, 515, 520, 525, 530 and 535. This is becauseapplying a pulse of V_(W)′ in one direction (+V_(W)′) will decrease theresistive state of the CMO material 225 from R₀ to R₁ and applying apulse of V_(W)′ in the opposite direction (−V_(W)′) will increase theresistive state from R₁ to R₀.

The polarity of the voltage drop is not important during a read. Theresistive state of the memory element can be detected regardless ofwhether the x-direction conductive array line 305 is V_(R)′ higher orV_(R)′ lower than the y-direction conductive array line 310. However, itis probably desirable to alternate polarities of reads so the memoryelements will not be subjected to any long-term read disturbs.

Additionally, a write operation would usually be preceded by a readoperation in order to ensure a write is necessary. In other words,+V_(W)′ should only be applied to a memory plug in its R₀ resistivestate and −V_(W)′ should only be applied to a memory plug in its R₁resistive state. By ensuring that a voltage pulse is only used when theCMO material 225 needs to change state, the CMO material 225 would noteither be placed in a resistive state lower than R₁ or higher than R₀ orsuffer any degradation that occurs with certain CMO materials.Additionally, avoiding a write operation will reduce disturbing theunselected cells and improve the endurance of the selected cell,avoiding unnecessarily stressing the cross point memory array 100.

If a read operation is used prior to a write operation, the polarity ofthe read should be the same as the polarity of the write operation thatwould follow, if required. For example, if the data to be written is “0”then the selected x-direction conductive array line 305 would be 3V andthe selected y-direction conductive array line 310 would be −3V. If theread operation preceding the write used +2V on the selected x-directionconductive array line 305 and −2V on the selected y-direction conductivearray line 310, then the circuitry would only need to switch IV for bothconductive array lines 305 and 310. This can be preferable to thealternative (−2V on the selected x-direction conductive array line 305and +2V on the selected y-direction conductive array line 310), whichwould require the voltage to switch a total of 5V.

The ability of the CMO material 225 to be placed into several differentresistive states can be exploited to store more than one bit ofinformation per memory plug 205. By adjusting the pulse width andvoltage height of the voltage pulses that change the resistive states ofthe CMO material 225, multiple resistive states can be used. Forexample, the CMO material 225 might have a high resistive state of R₀₀,a medium-high resistive state of R₀₁, a medium-low resistive state ofR₁₀ and a low resistive state of R₁₁. A sensitive reading mechanismmight be able to distinguish between three or more bits of informationcontained in the CMO material 225.

A benefit of some CMO materials is that the characteristic hysteresisallows non-destructive reads. As shown in FIG. 8A any voltages between−V_(R) (−1 volt) and +V_(R) (1 volt) will have no effect on theresistive state of the memory element (i.e., it will remain at either100 kΩ or 1MΩ). Therefore, a write operation is not necessary after aread operation for such materials.

As shown in FIG. 8A, the hysteresis has two write threshold voltages,one for when the CMO material 225 is in the low resistive state and onefor when the CMO material 225 is in the high resistive state. In the lowresistive state, the first write threshold voltage is the point abovewhich any voltages applied across the CMO material 225 havesubstantially no effect on the resistive state of the CMO material 225and below which a voltage pulse will alter the resistance of the CMOmaterial 225. Similarly, in the high resistive state, the second writethreshold voltage is the point below which any voltages applied acrossthe CMO material 225 have substantially no effect on the resistive stateof the CMO material 225 and above which a voltage pulse will alter theresistance of the CMO material 225.

When initially fabricated, the CMO material 225 may be in a highresistive state. The CMO material 225 then transitions from its highresistive state to a low resistive state in response to a voltage pulse.Whether the x-direction conductive array line 305 is at +3V and they-direction conductive array line 310 is at −3V in order to lower theresistive state or vice-versa depends upon the specific properties ofthe material that is used. FIG. 8A is an example of the CMO material 225that requires a +2V pulse to lower its resistive state and FIG. 8B, amirror image of FIG. 8A, is an example of the CMO material 225 thatrequires a −2V pulse to lower its resistive state.

It should be noted that the actual write voltage that is used on the CMOmaterial 225 is greater in magnitude than the write threshold voltage.The actual write voltage is a voltage pulse that would place the CMOmaterial 225 into the desired resistive state, which may be lower thanthe highest physically attainable resistive state and higher than thelowest attainable resistive state. However, in FIG. 8A the appropriatewrite voltage for the previously described exemplary CMO material 225 is−2V when it is in its low resistive state and 2V when it is in its highresistive state.

Ideally, the CMO material 225 should switch very quickly from oneresistive state to another. For current applications, anything less than50 nanoseconds would be an appropriate switching speed. Additionally,once the CMO material 225 is placed in a resistive state, it should beable to retain that state for long periods of time. Ideally, thematerial should retain its resistive state for over ten years. Since theread voltage should not affect the resistive state, repeated applicationof the read voltage over ten years should not change the resistive stateof the CMO material 225.

Generally, the chemical and materials properties of the memory elementare selected to meet the electrical specifications set forth above. Forexample, the material preferably has a resistivity of not greater thanabout 1 ohm-cm, exhibits a change in resistance of at least about 10 x,and has this resistance change triggered by the application of a voltagepulse of not longer than about 100 ns duration and not greater thanabout than 3V in magnitude. In addition, the memory element materialshould be compatible with the requirements imposed by the generalfabrication process of the entire high density RAM. Of course, there issome flexibility in the process constraints. Thus, process and designengineers will have certain ranges of chemical, materials, and processparameters available to tailor for the specific goals at hand. Amongthese parameters are the annealing conditions, the depositiontemperature and method, and the material stoichiometry and thethickness.

To allow rapid access times (on the order of tens of nanoseconds) insmall dimension devices (on the order of hundreds of nanometers), thememory element material should have a resistivity of not more than about1 ohm-cm. It is known that the resistivity of the complex metal oxidesdepends upon various factors, often including some of the following:film thickness, oxygen content of the film, stoichiometry, elementalcomposition, deposition method and conditions, degree of crystallinity,crystallite size, crystalline orientation, and doping level and choiceof dopant. Current research indicates that suitably low resistivity(less than or equal to 1 ohm-cm) CMO materials can be achieved byjudicious choice of these parameters.

One example of a suitable film thickness for the memory applications ofthis invention is approximately 1000 Å to 3000 Å. Thinner filmssometimes have higher strains, usually resulting from a slightmisalignment with the seed layer, which can result in higherresistivity. Film thickness has been discussed by S. I. Khartsev, et al.in “Colossal magnetoresistance in ultrathin epitaxialLa_(0.75)Sr_(0.25)MnO₃ films,” Journal of Applied Physics, Vol. 87, No.5, 1 Mar. 2000, which is hereby incorporated by reference for allpurposes.

Another factor that affects resistivity is the oxygen content of thefilm. By properly controlling the exposure to oxygen during depositionand annealing (if any) the resistivity can be controlled. It has beenobserved that 1500 Å lanthanum manganese oxide (LMO) films deposited bypulsed laser deposition in oxygen environments have a lower resistivitythan films deposited in vacuum (but otherwise identical conditions). SeeY. G. Zhao, et al., “Effect of oxygen content on the structural,transport, and magnetic properties of La_(1-δ)Mn_(1-δ)O₃ thin films,”Journal of Applied Physics, Vol. 86, No. 11, 1 Dec. 1999, which ishereby incorporated by reference for all purposes. Cooling down thefreshly deposited film in an oxygen-containing atmosphere furtherdecreases film resistivity.

It has been further observed that adjusting the relative amounts of therare earths and the alkaline earth metals can modify resistivity. Higherratios of alkaline earth to rare earth metals can, to a degree (e.g., upto about 50:50 in lanthanum calcium manganese oxides), lowerresistivity. See Guo-Qiang Gong, et al., “Colossal magnetoresistance of1000000-fold magnitude achieved in the antiferromagnetic phase ofLa_(1-x)Ca_(x)MnO₃,” Applied Physics Letters, Vol. 67, No. 12, 18 Sep.1995, which is hereby incorporated by reference for all purposes.

Further, it has been found that some polycrystalline materials may havelower resistivities than their amorphous and single crystalcounterparts. It has also been observed that magnitude of the resistancechanges in single crystal CMO films exceeds that of the polycrystallinefilms. Large changes (i.e., greater than about 10x), are, however, nottypically necessary for making a practical memory chip.

In light of the above, some specific CMO materials suitable for use withthis invention will have the following properties: (1) the thickness ofthe deposited complex metal oxide film is between 1000 Å and 3000 Å; (2)the deposition and cool down and post deposition annealing (if any) isperformed in an oxygen rich ambient; (3) the ratio of the rare earth andalkaline earth metals is adjusted for lowest resistivity, e.g., about.5; (4) the material is deposited or annealed to give a polycrystallinestructure; (5) the material is deposited or annealed to increase thepercentage of crystallites in a preferred orientation; and (6) the CMOmaterial is doped with a material that has the effect of pinning theoxygen vacancies. Regarding the last property, it has been found that asmall percentage of chrome doping also has a beneficial effect onreliability and endurance of some CMO films.

In addition to the above properties, certain process and design featuresare important. First, the seed layer or other “substrate” on which theCMO deposits impacts the resistivity of the CMO and other properties.Often the underlying crystrallographic orientation of the substrate willepitaxially propagate to the upper levels the CMO element. So, forexample, if the underlying substrate has a 100 orientation, then the CMOmay preferentially deposit in a 100 orientation. Preferably, theunderlying substrate is a conductive electrode such a noble metal (e.g.,platinum) or relatively conductive CMO such as LaNiO₃. Inpolycrystalline structures, suitable crystallite sizes may range fromabout 100 Å to about 500 Å.

The general process operations for creating a complex metal oxide memoryelement include (1) providing a substrate on which to directly form thememory element, (2) depositing the memory element material on thesubstrate, and optionally (3) post-processing the deposited material toimpart a desired property. As indicated above, the substrate materialshould be polycrystalline or single crystalline, be conductive, andserve as an electrode.

Various physical vapor deposition (PVD) and chemical vapor deposition(CVD) techniques may be employed. Many forms of PVD and CVD can beemployed, assuming that they operate at temperatures compatible with theoverall device fabrication technology. Post-processing operations mustalso meet the temperature strictures of the technology. Often, thismeans that the deposition process should operate at temperatures below600° C.

Although thin-film processes such as facing target sputtering and laserannealing after deposition might allow for the formation of acrystalline memory element at temperatures low enough to use copper oraluminum conductive array lines, most thin-film processes would requiretemperatures of about 600° C. to form crystalline memory elements.Therefore, any layers of conductive array lines underneath the memoryelements would need to withstand those temperatures.

As an example, conductive array lines of tungsten (W) will withstandhigh temperature processes. FIG. 9 is an exemplary flow chart of variousprocessing steps that could be involved in a W cross point array. FIG.10 is an elevation view of a cell 1000 formed with the processing steps.At 905, standard front end of line (FEOL) processes can be used to formthe active circuitry that drives the cross point memory array. FEOLprocesses are generally defined as operations performed on asemiconductor wafer in the course of device manufacturing up to firstmetallization, and might end with chemical-mechanical polishing (CMP) ofan inter-layer dielectric (ILD) 1002, such as SiO₂.

Regardless of the FEOL process, the next processing step at 910 isformation of contact holes through the ILD 1002 to appropriate positionsin the circuitry followed by W plug 1005 formation at 915. Abarrier/adhesion layer 1010 of 100 Å of Ti followed by 200 Å of TiNcould be sputtered on the wafer, followed by 5000 Å of W deposited usingCVD, followed by etchback or CMP to remove W on the ILD surface 1002,leaving W plugs 1005 in the contact holes.

Once the plugs are formed, the W conductive array lines 1020 arepatterned on the wafer at 920. Since W has a relatively highresistivity, the maximum length and minimum cross-sectional area may belimited in comparison to aluminum or copper. Specifically, the maximumlength and cross-sectional area of the conductive array lines 1020 canbe determined using $R = \frac{\rho\quad L}{A}$and setting a maximum resistance to about 10 kΩ in order to maintainfast access times. Assuming 5000 Å thick metallization layers and afeature size of 0.25 μm, the length of the conductive array lines 1020would be a maximum of about 2000 μm long. The W conductive array lines1020 can be achieved through a barrier/adhesion layer 1015 of 100 Å ofTi plus 200 Å of TiN, followed by 5000 Å of W deposition through CVD,followed by mask, etch, and resist strip steps.

Another ILD layer 1025 could be deposited over the first layer ofconductive array lines at 925. The dielectric layer 1025 can be a thicklayer of SiO₂, deposited over the W conductive array lines 1020 byplasma-enhanced chemical vapor deposition (PECVD) and then planarized byCMP to expose the top surfaces of the W lines 1020.

At step 930 the bottom electrodes 1030 are deposited. First, a 500 Åthick barrier layer of TiN is deposited to prevent metalinter-diffusion, followed by a 200 Å seed layer of LaNiO₃ (LNO) orSrRuO₃ (SRO). These layers can be deposited by sputtering.

At step 935 approximately 2000 Å of memory material 1035 having astoichiometry of Pr_(0.7)Ca_(0.3)MnO₃ is deposited at about 600° C. by aphysical vapor deposition technique such as sputtering. As previouslyexplained, the memory element 1035 would have a low resistance of 100 kOhm and a high resistance of 1M Ohm, and would change state with a lessthan 50 ns flat pulse at 2V. At 940 another electrode 1040 (200 Å of LNOor SRO and another 500 Å of TiN are deposited via sputtering) isdeposited on top of the memory element 1035.

At 945 through 955 the non-ohmic device 1045 is formed. The device 1045can be formed by first sputtering 250 Å of Al, followed by 50 Å ofAl₂O₃, and another 250 521 of sputtered Al. The Al₂O₃ could be formed byatomic layer deposition (ALD) or oxidization of Al. The Al₂O₃ thicknesswould be chosen to achieve a V_(NO+) of 4V. After the non-ohmic device1045, another 500 Å barrier layer 1050 of sputtered TiN is optionallydeposited in step 955 to prevent metal inter-diffusion.

At 960 standard photolithography and appropriate multi-step etchprocesses could be used to pattern the memory/non-ohmic film stack intomemory cell plug. At 965 the spaces between the plugs could then befilled in by depositing a 250 Å etch stop/diffusion barrier 1055 ofSi₃N₄, followed by a thick SiO₂ interlayer dielectric (ILD) 1060, whichis planarized by CMP.

At 970 via holes are formed. Via holes with W plugs could be formed toprovide connections between metal interconnect layers. Standardphotolithography and via etch could be used to make via holes. These viaholes could be filled by depositing 100 Å of Ti, followed by 200 Å ofTiN, followed by a 5000 Å W layer. CMP could then be used to remove W onthe ILD surface 1060, leaving the W plugs in the via holes.

If there are no more memory elements to form at high temperature, thefinal layer of conductive array lines may comprise aluminum, copper orother high conductivity metal. A top layer of conductive array linescould then be formed at 980 by depositing, in order, anotherbarrier/adhesion layer 1065 of 100 Å of Ti and 200 Å of TiN, then theconductive array line 1070 comprising 5000 Å of an Al/Cu mixture, andthen a final barrier/adhesion layer 1075 of 100 Å of Ti, 200 Å of TiN.An anti-reflective coating (ARC) 1080, such as SiON could also bedeposited. A final mask, etch and resist strip would then be performed.The final cross-point memory array could then be 16384 W conductivearray lines by 4096 Al/Cu conductive array lines to create a 64 Mbitarray. 16 of these arrays could be laid side-by-side to create a 1 Gbitmemory.

Note that the above example assumes that memory elements are formed by ahigh temperature process that could be incompatible with conventionalaluminum and copper deposition processes. Other processes for formingmemory elements at low temperatures exist. These include facing targetsputtering and laser annealing.

Although the invention has been described in its presently contemplatedbest mode, it is clear that it is susceptible to numerous modifications,modes of operation and embodiments, all within the ability and skill ofthose familiar with the art and without exercise of further inventiveactivity. Accordingly, that which is intended to be protected by LettersPatent is set forth in the claims and includes all variations andmodifications that fall within the spirit and scope of the claim.

1. A cross point memory array comprising: a first layer of conductivearray lines, the conductive array lines being arranged so that they donot come into direct contact with each other; a second layer ofconductive array lines, the conductive array lines being arranged sothat they do not come into direct contact with either each other or anyof the conductive array lines of the first layer; a plurality of memoryplugs located at the intersections of the first layer of conductivearray lines and the second layer of conductive array lines, each memoryplug being in electrical contact with one of the conductive array linesfrom the first layer and one of the conductive array lines from thesecond layer such that each memory plug is associated with a unique pairof conductive array lines; having a low resistive state and a highresistive state, the resistive state of the memory plug capable of beingdetermined when a read voltage is applied across the unique pair ofconductive array lines; and exhibiting a hysteresis that ischaracterized by a first write threshold when the memory plug is in thelow resistive state and a second state threshold when the memory plug isin the high resistive state, wherein voltages applied across the uniquepair of conductive array lines that are higher than the first writethreshold have substantially no effect on the resistive state of thememory plug when the memory plug is in the low resistive state; andvoltages applied across the unique pair of conductive array lines thatare lower than the second write threshold voltage have substantially noeffect on the resistive state of the memory plug when the memory plug isin the high resistive state.
 2. The cross point memory array of claim 1,wherein the read voltage is higher than the first write thresholdvoltage and lower than the second write threshold voltage.
 3. The crosspoint memory array of claim 1, wherein voltage pulse applied across theunique pair of conductive array lines that is suitable to change thememory plug from a low resistive state to a high resistive state canchange the resistive state of the memory plug in less than 50nanoseconds.
 4. The cross point memory array of claim 1, wherein avoltage pulse applied across the unique par of conductive array linesthat is suitable to change the memory plug from a high resistive stateto a low resistive state can change the resistive state of the memoryplug in less than 50 nanoseconds.
 5. The cross point memory array ofclaim 1, wherein repeated application of the read voltage over ten yearswill not change the resistive state of the memory plug.
 6. The crosspoint memory array of claim 1, wherein the resistive state will bemaintained for a minimum of ten years if no voltage is applied acrossthe unique pair of conductive array lines that is lower than the firstwrite threshold or higher than the second write threshold.
 7. A methodcomprising: selecting a unique pair of conductive array lines associatedwith the single memory plug, the unique pair consisting of a first layerconductive array line and a second layer conductive array line, whereinthe memory plug exhibits a hysteresis that is characterized by a firstwrite threshold voltage, above which voltages applied across the uniquepair of conductive array lines have substantially no effect on theresistive state of the memory plug when the memory plug is in the lowresistive state; and a second write threshold voltage, below whichvoltages applied across the unique pair of conductive array lines havesubstantially no effect on the resistive state of the memory plug whenthe memory plug is in the high resistive state.
 8. The method of claim7, further comprising applying a read voltage across the unique pair ofconductive array lines, wherein the read voltage is above the firstwrite threshold voltage and lower than the second write thresholdvoltage.
 9. A cross point memory array comprising: a first layer ofconductive array lines, the conductive array lines being arranged sothat they do not come into direct contact with each other; a secondlayer of conductive array lines, the conductive array lines beingarranged so that they do not come into direct contact with either eachother or any of the conductive array lines of the first layer; aplurality of memory plugs located at the intersections of the firstlayer of conductive array lines and the second layer of conductive arraylines, each memory plug being in electrical contact with one of theconductive array lines from the first layer and one of the conductivearray lines from the second layer such that each memory plug isassociated with a unique pair of conductive array lines: having a lowresistive state and a high resistive state, the resistive state of thememory plug capable of being determined when a read voltage is appliedacross the unique pair of conductive array lines; and exhibiting ahysteresis that is characterized by a first write threshold when thememory plug is in the low resistive state and a second write thresholdwhen the memory plug is in the high resistive state, wherein voltagesapplied across the unique pair of conductive array lines that are lowerthan the first write threshold have substantially no effect on theresistive state of the memory plug when the memory plug is in the lowresistive state; and voltages applied across the unique pair ofconductive array lines that are higher than the second write thresholdvoltage have substantially no effect on the resistive state of thememory plug when the memory plug is in the high resistive state.
 10. Thecross point memory array of claim 9, wherein the read voltage is lowerthan the first write threshold voltage and higher than the second writethreshold voltage.
 11. The cross point memory array of claim 9, whereina voltage pulse applied across the unique pair of conductive array linesthat is suitable to change the memory plug from a low resistive state toa high resistive state can change the resistive state of the memory plugin less than 50 nanoseconds.
 12. The cross point memory array of claim9, wherein a voltage pulse applied across the unique pair of conductivearray lines that is suitable to change the memory plug from a highresistive state to a low resistive state can change the resistive stateof the memory plug in less than 50 nanoseconds.
 13. The cross pointmemory array of claim 9, wherein repeated application of the readvoltage over ten years will not change the resistive state of the memoryplug.
 14. The cross point memory array of claim 9, wherein the resistivestate will be maintained for a minimum of ten years if no voltage isapplied across the unique pair of conductive array lines that is higherthan the first write threshold or lower than the second write threshold.15. A method comprising: selecting a unique pair of conductive arraylines associated with the single memory plug, the unique pair consistingof a first layer conductive array line and a second layer conductivearray line, wherein the memory plug exhibits a hysteresis that ischaracterized by a first write threshold voltage, below which voltagesapplied across the unique pair of conductive array lines havesubstantially no effect on the resistive state of the memory plug whenthe memory plug is in the low resistive state, and a second writethreshold voltage, above which voltages applied across the unique pairof conductive array lines have substantially no effect on the resistivestate of the memory plug when the memory plug is in the high resistivestate.
 16. The method of claim 15, further comprising applying a readvoltage across the unique pair of conductive array lines, wherein theread voltage is lower than the first write threshold voltage and higherthan the second write threshold voltage.